When air goes into a centrifugal pump, it can cause the pump to stop pumping water, because the centrifugal impeller cannot efficiently pump air and suck water from a supply pipe. If the water source is far away from the pump, it cannot efficiently pump water until the entire length of the source pipe is filled with water. A pump full of air may not be able to generate enough pressure difference to draw water into the pump. If some air bubbles are present in the long pipe, these air bubbles can combine to form an air pocket which can stop the pumping of water. Therefore it is common practice to position centrifugal pumps close to the water source (e.g., a river or lake). In the case of wells, the pump is positioned at the bottom of the well to make the distance between the centrifugal pump and the water source as short as possible.
There is also a risk, when putting an expensive diesel powered centrifugal pump a mile or more away from a house or farm, that the pump can be stolen at night. If the pump is electric, a long cable would be needed extending from the house to the water source. The cable installation can be expensive, and it, too can be stolen. Further, if maintenance is required, it may be necessary to travel a long way, perhaps in the middle of the night, to correct a simple problem, for example with fuel supply to the diesel engine.
Even with a pump positioned close to the water source, a “priming” operation is often required. In many pumps, this process is achieved manually by pouring liquid into the pump through a priming port. If there is a lot of air that must be purged from the supply line, either enough water must be added to completely fill the supply line, or the priming operation may need to be repeated several times until all of the air is pumped out of the supply line. Typically, a small amount of air can be “pumped” by entraining it into pumped water, but larger amounts can fill the pump with air again and cause it to stop pumping. “Self-priming” pump designs are also known, whereby the priming operation is automated in some way. For example, in some installations, it may be possible to position the pump below the water level (either literally submersed, or adjacent to the water supply but at a level below the water surface). In these installations, gravity can provide the force required to keep the pump primed, even if the supply pipe is leaky, and the pump provides the additional force required to pump water to a higher elevation and/or a distant location.